This invention relates to controlled-porosity dispenser cathodes for electron tubes and especially to a method and means for shadowing the grid of controlled-porosity dispenser cathode tubes so that emitted electrons will not strike the grid.
In many high-power microwave tubes, it is necessary to turn the electron beam on and off by means of a control electrode (grid) spaced very close to the cathode surface. By varying the voltage on this electrode, electron emission from the cathode surface can be modulated. Because of the electrical field configuration in many of these power tubes, it is necessary to operate the control grid at a positive potential with respect to the cathode in order to turn the beam on. In this event, it is necessary to have a shadow grid between the cathode surface and the control grid to prevent excessive electron current from being intercepted by the control grid (see FIG. 1). Excessive current in the control grid circuit requires excessive power capabilities from the control grid power supply. Using present state-of-the-art practice, the control grid/shadow grid combination is achieved in two ways:
(1) A control grid/shadow grid combination is constructed as an integral unit and the unit is spaced as an integral unit and the unit is spaced over the cathode surface, as shown in FIG. 1; PA1 (2) An integral shadow grid is placed directly on the cathode surface, either by making a separate grid structure and laying it on the surface, or by depositing a metallic layer on the cathode surface in a pattern that will permit registry by the separate control grid structure. In this case, the metallic layer on the cathode surface must be of a material that will maintain an electronically non-emitting state when at cathode temperature and when exposed to various emission-enhancing materials evolving from the cathode. There are disadvantages to both these methods. PA1 a. Most dispenser cathodes are made by sintering a pressed plug of tungsten powder to create a porous matrix which is impregnated with a barium compound. Consequently, the cathode has a textured surface with a random distribution of pores. At cathode operating temperatures, the barium compound migrates out of these pores and normally forms a low-work-function film on the emitting surfaces surrounding the pores. The coating deposited on the shadow grid areas is usually of zirconium or some similar metal from which the barium compound will evaporate readily at normal cathode operating temperatures. However, since the coating is deposited directly on some pore areas of the cathode, some barium compound will evolve directly through defects in the coating and, in some cases, the coating will not adhere well to the non-metallic material in the pores (see FIG. 2). In both cases, localized sources of emission will result in the shadow grid areas giving rise to undesirable control grid currents. PA1 b. With the currently used random-porosity dispenser cathodes, the shadow grid must be deposited on the cathode surface after the cathode has been impregnated with the barium compound which forms the activating surface layer. Since the cathode is very susceptible to poisoning by foreign materials, care must be taken about the type of environment the cathode is exposed to after impregnation. This limits the methods that can be used to apply the shadow grid material. For example, chemical vapor deposition and electrodeposition techniques, which are often used to deposit uniform well-adhering coatings of refractory materials, cannot be used because of cathode contamination risks.
Method (1) requires a very precisely made assembly spaced very close to the cathode surface. The amount of voltage swing required on the control grid to modulate the beam current increases with increased spacing between the grids and also with the spacing between the grid assembly and the cathode. Increased spacing also limits the speed at which the beam can be turned on and off. Because of differential thermal expansion effects and limitations in the precision of construction methods, reproducibility and stability of spacings is a major problem.
Method (2) is an improved method insofar as it overcomes the spacing problem between the shadow grid and cathode by depositing the shadow grid directly on the cathode. Thus, it gives rise to faster turn on and off characteristics, and less susceptibility to thermal expansion problems. However, having the shadow grid directly in contact with the emitting surface of the cathode and operating essentially at the cathode temperature can cause other problems. For example, such a shadow grid will emit electrons if it cannot maintain a high work function. Other problems include: